There is a known impact type dot printer as disclosed in U.S. Pat. No. 3,629,504. The known printer comprises a print head mounted on a carriage which is movable along guide shafts relative to a rotary platen. The platen has a plurality of angularly spaced-apart projections or ridges which coact with a print hammer of the print head to print dot-matrix characters on a recording medium situated between the print hammer and the platen. The print hammer confronts one of the projections substantially in a crossing manner and is selectively actuated when it confronts the selected projection of the rotary platen at proper positions corresponding to the respective row positions of a dot-matrix of characters to be printed. In such a printer, it is very difficult to properly control the printing operation since the positions of printed dots are determined by the relative positional relationship between the print hammer and the projections of the platen. Accordingly, the known printer is adapted to progressively print each symbol or character during each printing cycle and, to synchronize the initiation of each printing cycle with the position of the projections of the platen, it requires sensing means for sensing the position of the projections of the platen and means responsive to the sensing means for controlling initiation of each printing cycle and for thereafter controlling the print hammer to print at each predetermined grid block location until each entire symbol has been printed. Further, there is actually required to employ two transducers for controlling the printing operation.